Electrochemical analysis is a highly sensitive, chemically selective method for identifying and quantifying many different chemicals in water. Sub-part-per-billion sensitivity levels are achievable for many EPA-regulated chemicals and for many of the chemicals proposed for future regulation. Unfortunately, electrochemical analysis has historically required that field samples be transported to an analytical laboratory where additional laboratory chemicals must be added to the samples before an electrochemical analysis can be performed with bench-scale equipment. This requires skilled laboratory technicians and causes unacceptable time delays when immediate information about the safety of a particular water source is needed. Furthermore, sample degradation may occur during sample transport to the laboratory, thereby causing further uncertainty in the analytical results. Conventional off-site laboratory analysis is also too costly if comprehensive, continuous monitoring of the health of a water distribution system is desired.
The necessary addition of chemicals for currently available electrodes to work well with many water samples is a key limiting factor preventing on-site, real-time measurements and distributed unmanned operation. Due to their size, the macroscale working electrodes most often used in electroanalysis today require the addition of an electrolyte to the solution to adjust the conductivity, ionic strength, and/or pH of the solution before an accurate electrochemical analysis can be made. Recently, there has been emphasis on scaling down the working electrode to microscale dimensions to achieve geometry related increases in the diffusion-limited current density. Unfortunately, the incorporation of microelectrodes in analytical methods is severely hampered by the small faradaic currents (i.e., small analytical signals) typical for these electrodes. Furthermore, microelectrodes also require supporting electrolytes for proper operation. For example, a 1-μm-diameter working electrode typically requires that a supporting electrolyte solution be added to achieve a minimum resistivity of 80-100 Ω-cm. This necessitates the presence of a trained technician to properly adjust solution concentrations.
Further reduction of the electrode dimension from the microscale to the nanoscale can remove these resistivity limitations, enabling unattended operation or direct measurements by simple immersion in a water source without any electrolyte addition. However, because the faradaic current from an individual nanoelectrode is very small, a massive array of nanoelectrodes is required to obtain an adequate current signal. Furthermore, integrated nanoelectrode arrays and portable control electronics are needed to enable reliable electrochemical measurements to be made in the field. Such integrated nanoelectrode arrays are not readily available using current fabrication techniques. The nanoelectrode array of the present invention overcomes the limitations of current electrode designs and can enable both portable, battery-powered field testing and continuous remote system monitoring.